Date: Thu, 12 Oct 2017 13:55:18 +1100 From: "Tobin C. Harding" <me@...in.cc> To: Theodore Ts'o <tytso@....edu>, kernel-hardening@...ts.openwall.com, linux-kernel@...r.kernel.org, Linus Torvalds <torvalds@...ux-foundation.org>, Kees Cook <keescook@...omium.org>, Paolo Bonzini <pbonzini@...hat.com>, Tycho Andersen <tycho@...ker.com>, "Roberts, William C" <william.c.roberts@...el.com>, Tejun Heo <tj@...nel.org>, Jordan Glover <Golden_Miller83@...tonmail.ch>, Greg KH <gregkh@...uxfoundation.org>, Petr Mladek <pmladek@...e.com>, Joe Perches <joe@...ches.com>, Ian Campbell <ijc@...lion.org.uk>, Sergey Senozhatsky <sergey.senozhatsky@...il.com>, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will.deacon@....com>, Steven Rostedt <rostedt@...dmis.org>, Chris Fries <cfries@...gle.com>, Dave Weinstein <olorin@...gle.com>, Daniel Micay <danielmicay@...il.com>, Djalal Harouni <tixxdz@...il.com> Subject: Re: [PATCH] printk: hash addresses printed with %p Removing kvm@...r.kernel.org from the CC list. On Wed, Oct 11, 2017 at 01:48:58PM -0400, Theodore Ts'o wrote: > On Wed, Oct 11, 2017 at 02:48:16PM +1100, Tobin C. Harding wrote: > > +/* > > + * Obfuscates pointer (algorithm taken from kptr_obfuscate(). See kernel/kcmp.c) > > + * v is the pointer value, randval is some random value, oddval is some random > > + * odd value. > > + * > > + * The obfuscation is done in two steps. First we xor the kernel pointer with > > + * a random value, which puts pointer into a new position in a reordered space. > > + * Secondly we multiply the xor production with a large odd random number to > > + * permute its bits even more (the odd multiplier guarantees that the product > > + * is unique ever after the high bits are truncated, since any odd number is > > + * relative prime to 2^n). > > + */ > > Why not just expose kptr_obfusecate() and use it, instead of copying > code? > > Also, I'm nervous about the obfuscation. If the attacker can get a > handful of known "real kernel pointer" and "obfuscated kernel pointer" > values, it wouldn't be that hard for them to be able to reverse > engineer the two secret values. > > Perhaps the argument is "if the attacker can get a _single_ real > kernel address, it's all over anyway", which is probably true for > KASLR, but which might not be true for all attacks. > > Anyway, if you use kptr_obfuscate in kernel/kcmp.c, then if we later > decide that we should change the obfuscation algorithm to something > stronger, we only need to do it in one place. > > - Ted Thanks Ted, others have misgivings about this method also. The email threads are all a bit mixed up (thansk to my ineptness at posting patches :). FYI, in the other threads Jason A. Donenfel and Linus Torvalds have discussed SipHash as a solution. thanks, Tobin.
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